Civil's Guide
Structural Timber
Why design using structural timber
Structural timber is becoming a more common material in the design and construction of new buildings. This is primarily due to technological advances and the ease of producing high strength timber that can be used for columns, beams in high rise buildings. An example is a 14-stroey timber tower called “The Tree” was built in Norway using cross-laminated timber for floors and walls.
The differences between using timber and traditional materials are listed below:
Timber:
- Medium material cost
- Low maintenance
- Quick, clean construction
- Low self-weight
- Variable properties
- Good fire resistance
- Very low \(C0_2\) emission during processing
- Very good recyclability
Steel:
- Expensive material cost (depends on the price of steel)
- High maintenance in exterior use
- Quick, clean construction
- Low self-weight
- High, reliable strength
Concrete:
- Cheap
- low maintenance
- slow, messy construction
- High self-weight (need bigger foundations)
- Variable properties
Types of engineered Timber
- Glulam
- OSB (Oriented Strand Board
- LVL, (Laminated Vaneer Lamber)
- Cross laminated Timber (CLT)
- Wood polymer composites
- More homogenous with less defects
- Designable to meet specific applications
- Versatile and available in different geometry and grade
- Maixmise the natural strength and stiffness characteristics of wood
- Easy to work with using orginary tools and basic skills
- More efficient use of wood
- Requires more primary energy to produce
- The adhesives used may be toxic
- Specialist contractors may be needed
Glulam
Glulam is made up of laminated timber, which is glued (bonded) together to form a singe structural element. The grain of timber in each layer is parallel to all other layers. Glulam members can be used for structural glulam beam and column members.
It is commonly used as roof beams as large spanning structures in sports halls, swimming pools and also used as bridge beams as well as trusses.
Cross laminted timber (CLT)
Cross laminated timber consists of solid sawn timber, which are glued (bonded) together into a single structural member. The grain in each layer is perpendicular to those on either side. Common uses are one-way spanning floor slabs, wall panels and roof panels. This is a lightweight solution, when compared with a concrete floor as this could save cost and time through using smaller foundations. However, floor vibration could become an issue (gyms, dance floors) as well as acoustic issues.
Laminated Vaneered lumber (LVL)
LVL consist of thin sheets (veneers) of wood, laid out in the same orientation, coated with adhesive and hot pressed. Required member sizes can then be cut from the sheet. It is commonly used as structural beams and formwork. As they are made up of thin veneers, which reduces the effect of defects but the member sizes are generally limited to around 75mm (although larger members can be achieved by mechanically fixing/gluing panels together).
Durability and Fire Resistance of Timber
Decay
Timber decays normally occurs when the moisture content is above 20% for most of the year. This applies to internal timber members and exposed timber member if it is properly drained and ventilated.
However, engineers should design them so they do not get wet or there is adequate drainage.
Insect Attack
Insect attack is an issue to be looked at in the design of timber but the UK does not have serious issues with insect attack and the main problems come from fungal attack.
In the UK, insect attack mainly occurs from furniture beetles (woodworms), death watch beetles and the house longhorn beetle.
Fungal Attack
Structural timber can become susceptible to fungal attack when conditions are right for the fungi to grow when the moisture content increases to a minimum of 20%.
The two types of fungal attack are dry rot (most serious) and wet rot. Dry rot is caused by the ‘Serpula lacrymans’ which can spread and damage much timber throughout a structure. White cotton wool like strands form on the surface of the timber. Sometimes, yellow patches can be seen. This fungus weakens the timber, causing it to be dry and brittle.
A serious issue is that it can spread between newly built timber and so the true of extent of structural damage may be larger than imagined.
Wet rot is caused by ‘Coniophora puteana’. This is a common cause of timber decay and is usually confined to areas where there is dampness or leaks.
This occurs when moisture levels are between 30-50%. This is identified by dark brown staining on the timber as well as splitting along the grain of timber.
Fire Resistance
Timber has good resistance against fire and despite it being a combustible material, as it has natural insulating properties. When timber is exposed to fire, timber starts to break down into charcoal, carbon monoxide and methane. The charcoal layer forms around the timber and acts as
insulation which protects the inner core of the timber which maintains the strength and stability of timber members.
The ISTRUCTE guide for the design of timber members has predicted rates for charring of timber, which is tabulated below:
| Material | Charring rate per exposed face (mm/min) |
|---|---|
| Softwood timber | 0.8 |
| Softwood glulam, and LVL to BS EN 14374 | 0.7 |
| Hardwood timber and hardwood glulam with a characteristic density of \(\geq 450\) \(kg/m^3\) | 0.55 |